Under normal physiological conditions, the pH of plasma and tissues is maintained at values slightly above neutral pH, in a very narrow range of pH values from approximately 7.38 to 7.42. Some pathological conditions may lead to a systemic decrease in pH such as metabolic acidosis which can be caused by diabetic ketoacidosis, alcoholic ketoacidosis, ketoacidosis due to starvation, poisonings (e.g., methanol, ethylene glycol, salicylates , etc.), severe diarrhea, enzyme defect, and the like. All of these conditions can result in a decrease in systemic pH, although not below pH 7.0, even in severe cases. A similar decrease can be observed in respiratory acidosis that can be caused by decreased ventilation, whether acute or chronic.
In addition to disease conditions that result in a systemic decrease in pH, there are many diseases in humans that produce a localized decrease in pH. These conditions include a wide variety of infectious diseases, as well as many tumors which are related to hypermetabolic activity and/or hypoxic state, all of which are capable of inducing the phenomenon of a localized decrease in normal physiological pH. In localized infectious diseases, the pH can be as low as 4.5, whereas in tumor sites, the pH is 0.7 to 1.0 pH unit lower than normal physiological pH.
All chemotherapeutic agents used to treat cancer are associated with severe side effects and toxicity phenomena, most of which are dose dependent. Most anti-infectious agents also demonstrate dose-dependent adverse side effects and toxicity. Therefore, it would be advantageous to be able to reduce these adverse effects by the use of a prodrug that imparts reduced toxicity to therapeutically active systems. Alternatively, it would also be advantageous to reduce the overall toxic effects of therapeutic agents on a patient's system through minimization of the delivery of the therapeutic, and therefore toxic, component of treatment agents to clinically irrelevant tissue sites.
It would also be advantageous to target diseased tissues characterized by slightly acidic pH values through utilization of a prodrug which could be activated selectively within a range of pH values falling below 7.0. It would thus be possible to introduce a therapeutic system comprising a prodrug with both a pharmacologically active component and an acid-labile linker component into a patient and selectively activate the therapeutic system by decomposing the prodrug and releasing the pharmacologically active component in the target diseased tissues rather than in the healthy cells. In this manner, delivery of the toxic, pharmacologically active component of the prodrug to tissue sites that do not meet the criteria of reduced pH associated with certain disease states, and where the presence of the active component is clinically irrelevant, can be minimized. This has the potential to improve the therapeutic index of the drug, and possibly lower the total dosage necessary to achieve the desired clinical result, thereby reducing the toxic side effects of the pharmacologically active species on the patient's system.
The use of prodrugs to impart desired characteristics such as increased bioavailability or increased site-specificity of known drugs is a recognized concept in the state of the art of pharmaceutical development. The use of various blocking groups, which must be removed in order to release the active drug, is also known in the prior art. Commonly, one or more blocking groups may be attached via an available reactive functional group on the drug. This type of prodrug may be cleaved by nonspecific esterases to effectively release the active principle over a prolonged period of time, in a sustained-release fashion compared to the native drug species. In none of the examples of the use of such a strategy known to the prior art has it been possible to achieve preferential accumulation of the drug within the diseased tissues or organs by activation after exposure to a specific range of pH outside of normal physiological values.
A wide range of blocking groups is known in the art, as compiled, for instance, in the textbook of Greene and Wuts, Protective Groups in Organic Synthesis, John Wiley & Sons, Inc.: New York (1991). As disclosed in the prior art, many blocking groups are reactive under strongly acidic conditions, i.e., pH values in the range of 1 to 2. However, only a few are marginally reactive in the range of pH between 4 and 6 (see, for example, ibid., pp. 411-450). Nowhere in this above referenced art is it taught or suggested that a blocking group for an amine or hydroxyl group, for example, can be removed efficiently by exposure to pH values in the range of about 4 to 7.
Acid labile protecting groups have been used also in the laboratory synthesis of nucleic acids and also peptide nucleic acid as disclosed for example in WO 96/40709, WO 94/22802, and WO 93/20090. These disclosures do not relate to the field of drug delivery or prodrug activation.
Acid-labile cis aconitic anhydride derived linker molecules for targeting cytotoxic therapeutic agents such as adriamycin, used in the treatment of cancer, are disclosed in U.S. Pat. No. 5,306,809. These linkers are designed to conjugate the drug to a carrier molecule such as a protein, an antibody or antibody fragment, a polymer, or a nucleic acid in order to provide a targeting system to the vicinity of the cancer cells where the pharmacologically active species of the therapeutic agent is then available to react with target tissue to which it is preferentially delivered on the basis of a property, such as immunospecificity in the case of antibodies, of the carrier molecule.
Conjugates constructed to release free drug at the lower pH of tumor tissue were also disclosed, using monoclonal antibody L6 and daunomycin (Lavie et al., Cancer Immunol, Immunother. 33, 223-230, 1991). This targeting system involved a pH sensitive linker of cis aconitic anhydride, which attached the drug to the antibody.
Acid-sensitive spacer molecules used in conjugates constructed to release the free drug at the lower pH of tumor tissue were also disclosed in U.S. Pat. Nos. 4,631,190, 4,997,913 and 5,140,013. This targeting system involved a drug linked to a biopolymer or an antibody via a pH sensitive linker based on cis aconitic anhydride. THe immunospecificity of the antibody segment of the system enables the preferential delivery of the conjugate to the target tissue site. Once there, the decreased pH conditions release the pharmacologically active component, where it then free to react with the tumor tissue.
Other types of prodrugs are known in the art to enable the preferential delivery of a drug to a specific target tissue site, or to be released within a specific organ. This approach is exemplified in the case of the phospholipid prodrugs of salicylates and nonsteroidal anti-inflammatory drugs disclosed in WO 91/16920 which, taken orally, protect the gastric mucosa and release the active drug ingredient in the gut. In other examples of phospholipid prodrugs, formulation of the prodrugs into liposomes or other micellar structures is the feature that enables their preferential uptake, for instance by macrophages, or by liver cells, as in the case of the phospholipid conjugates of antiviral drugs disclosed in WO 90/00555 and WO 93/00910. In other instances, specific types of polar lipids are used to target the prodrugs to intracellular organelles, as in the case of the antiviral and antineoplastic nucleosides disclosed in U.S. Pat. No. 5,149,794.
A novel esterase-sensitive cyclic prodrug system for peptides that utilizes a trimethyl lock-facilitated lactonization reaction for the release of the peptides has been disclosed in U.S. Pat. No. 5,672,584. This disclosure describes a strategy for preparing cyclic prodrugs of peptides that have increased metabolic stability and increased cell membrane permeability compared to the linear peptide. This strategy involves taking advantage of a predesigned lactonization system. In human plasma, the prodrug releases the original peptide through what is believed to be the esterase-catalyzed hydrolysis of the phenyl ester bond which initiates the lactonization reaction. As a result of this specific reaction, the linear peptide is reformed and released in the target cell. It is neither taught nor suggested in this reference that this type of prodrug species might be activated (converted to the active therapeutic form) by other than esterase catalyzed hydrolysis. The reference is utterly lacking in any teaching or suggestion that acidic pH induced-lactonization could achieve the desired end product of a linear peptide. More importantly, there is no teaching or suggestion in this reference that the specific cyclization scheme might have any utility for drugs other than peptides.